The invention relates to a sample rate converter.
Sample rate converters are known in the art and are used to convert a digital signal with a first (input) sample rate (sampling frequency) into a digital signal with a second (output) sample rate (sampling frequency). The sample rate can be increased (up-converter) or decreased (down converter).
Such sample rate converters are required when processing signals of a system using a first sampling rate in a system, which uses a second sampling rate. For example compact disc uses 44.1 kHz, digital audiotape uses 48 kHz and satellite broadcasting uses 32 kHz.
If the ratio between the original frequency and the target frequency is not a whole number, known sample rate converters need a very high intermediate conversion frequency. Further a multiplicity of circuits and filters use multiple stages and the number of sets of coefficients that are necessary grows in proportion to the intermediate frequency. Further more these methods do not allow different target frequencies to be selected using a single circuit.
In U.S. Pat. No. 5,349,548 and U.S. Pat. No. 5,625,581 non-integral delay circuits are described that can be used in such a sample rate converter. From U.S. Pat. No. 4,748,578 a sample rate converter is known which has a selectable output frequency, but with a high intermediate frequency.
From U.S. Pat. No. 5,559,513 a sample rate converter is known that tries to solve the complexity of known sample rate converters by using first an up-conversion with a factor N (the required resolution of the output signal) creating an intermediate signal, followed by a linear interpolation between two adjacent samples of the intermediate signal based on the ratio between the input and the output sample rate.
A disadvantage of this known sample rate converter is that the linear interpolator that is used herein has to calculate all samples at the high sample rate. Further a moving time averager is used to supply a sum signal to the linear interpolator.
An object of the invention is to overcome the disadvantages of the prior art and further to obtain a sample rate converter which both can be used as up-converter and as down-converter using the same circuit.
To this end a first aspect of the invention provides a sample rate converter as defined in claim 1.
The sample rate converter of the present invention has as advantages that it is only necessary to calculate the samples that are necessary instead of calculating all the samples as the prior art sample rate converters do. Further a moving time averager is not necessary.
This is achieved by realizing that by using the combination of polyphase decomposition filter means and interpolation means a very efficient flexible sample rate converter can be obtained.
A considerable smaller number of calculations per second are needed and a considerable smaller number of coefficients have to be calculated and stored.
Further the sample rate converter is able to handle arbitrary input and output sampling frequencies.
The only thing that has to be known beforehand is the required suppression of the so-called mirror spectra and the necessary relative bandwidth. On the basis of this information the filter-coefficients are chosen and these coefficients can be used for both the up-sampler as the down-sampler.
An embodiment of a sample rate converter according to the invention comprises the features of claim 2.
By adding an auxiliary up-converter in front of the series-arrangement, the operation of the flexible sample rate converter is further improved. By using the auxiliary up-converter the up-conversion can be splitted in more stages, which improves the performance.
An embodiment of a sample rate converter according to the invention comprises the features of claim 3.
An embodiment of a sample rate converter according to the invention comprises the features of claim 4.
By using the measures of the invention it is possible to use the flexible sample rate converter for both up- as for down-conversion.
Further the filters are beforehand determined on the basis of the required image suppression (mirror spectra) and necessary relative bandwidth.
Further embodiments are described in the other dependent claims.